| Processing Steps |
- Parameter or Variable: microplastic concentration (measured); Units: pieces/m3; Observation Category: in situ; Sampling Instrument: Manta net; Sampling and Analyzing Method: This study collected microplastic samples in the surface water, beach sediment, and crude salt from the northern Bay of Bengal, Bangladesh coast. Water samples: Surface water samples (n = 18) were collected from three stations, W1—near Moheshkhali Island (21° 30′ 45″ N and 91°59′ 14″ E), W2—between Moheshkhali and Sonadia Island (21° 28′ 59″ N and 91° 57′ 36″ E), and W3—near Sonadia Island (21° 27′ 45″ N and 91° 54′ 25″ E) of Moheshkhali Channel with three replicates in March 2019, and January 2020. A manta net (mesh size: 300 μm; width of the opening: 60 cm) was used to collect microplastic samples from the water surface. The net was deployed from the vessel’s side, around 3–4 m from the vessel. For about 15 min, the action was set to proceed in a straight direction at a speed of approximately 2 knots. Then, the vessel was stopped, and the manta net was pulled from the water. The net was carefully rinsed from the outer side with clean water from the boat’s water reservoir. The cod-end was removed safely, and the sample inside was sieved using a 300 μm mesh-sized sieve. The sieve was rinsed using 70% ethanol in a labeled glass jar with a glass funnel. The use of 70% ethanol is essential to preserve the sample. In addition, in visualizing the sample, ethanol helps to discolor the organisms; therefore, colorful plastic items become easier to find. Then, the samples were processed in the Interdisciplinary Institute for Food Security (IIFS) laboratory at Bangladesh Agricultural University (BAU). Samples were poured through the metal sieves (successively using 5 mm, and 0.3 mm sieves), and all the litter objects (natural and artificial) were air-dried in a closed dish. Microplastic particles were separated from water samples according to Masura et al. (2015) with slight modification. Briefly, sieved samples (< 5 mm sized) were taken in 1 L glass beakers and dried in an oven (Genlab OV/200/F/ DI, England) at 90 °C for 24 h. In each beaker, 20 mL of 0.05 M iron sulfate solution and 20 mL of 30% hydrogen peroxide were added, and the mixtures were kept at room temperature for 5 min. The mixtures were heated at 75 °C on a hotplate (AM4, Velp Scientifica, Italy) until gas bubbles were observed. If natural organic matter is noticeable, an additional 20 mL of 30% hydrogen peroxide was added. Approximately, 6 g salt (NaCl) was added per 20 mL of sample to raise the density of the solution (~ 5 M) and heated at 75 °C until the salt dissolved. The density separator was filled with the Wet Peroxide Oxidation solution (WPO), then covered loosely with aluminum foil. The WPO solution was poured into the density separator and covered loosely with aluminum foil. Solids settled in the separator were excluded, and floating solids were taken in a custom sieve (0.3 mm). The density separator was rinsed multiple times with distilled water to transfer all particles to the sieve. Microplastics were identified under a microscope (Olympus CX41, Japan) at 4× to 100× magnification. Beach sediment samples: Beach sediment samples (n = 60) were collected from two beaches, Laboni beach, LB (21° 26′ 05″ N and 91° 57′ 51″E), and Crab beach, CB (21° 19′ 47″ N and 92° 01′ 45″ E) with ten replicates/quadrats in March 2019, September 2019, and January 2020 during the lowest low tide. Two 100 m large transacts along the strandline/tideline were taken at each beach, and the distance between the two large transacts was approximately 0.5 km. Within every 100 m transact, five small quadrats (50 cm × 50 cm) were randomly selected for plastic debris sampling. Within a 5 cm depth in the quadrat, all the debris (natural and manufactured) was sieved successively using 5 mm, and 0.3 mm metal sieves. Sediment samples were placed into the 5 mm sieve using a trowel and washed through the sieve using filtered seawater. All items larger than 5 mm remained in the top sieve. Items < 5 mm were passed through the 5 mm sieve but stayed on the top of the 0.3 mm sieve. The items (> 0.3 mm) were transferred using a metal spoon from the sieves into a labeled glass jar for storage and transport. Then, a minimum amount of 70% ethanol was added to each sample jar to preserve the sample. Next, the samples were processed in the laboratory of IIFS, BAU. The sediment samples were sieved and air-dried in a closed dish. The contents were carefully transferred into a glass serving dish containing a premade filtered salt water solution (35 g/L) to extract the microplastics (< 5 mm) from the samples. Most microplastics floated as they were lighter in density than the salt solution. Then, floating plastic particles were separated by visual identification and using a magnifying glass. Afterward, potential microplastics were identified under a microscope (Olympus CX41, Japan) at 4× to 100×magnification. Crude salt: Three different crude salt (n = 27) samples (1 kg) with three replicates were collected from three separate salt beds (SB) at salt production sites (21° 42′ 01″ N and 91° 54′ 56″ E to 21° 42′ 03″ N and 91° 54′ 58″ E) of Moheshkhali Island, Cox’s Bazar in March 2019, September 2019, and January 2020. Microplastics extraction from crude salt samples was accomplished as follows. In brief, 200 g of crude salt was measured per replicate sample and transferred to a 2 L glass bottle. 200 mL of 30% hydrogen peroxide (H2O2) was added to each bottle to digest the organic matter. Then the bottles were covered with aluminum foil paper and placed in a shaking Water Bath (WiseBath, Witeg, Germany) at 65 °C with 80 rpm for 24 h. After 24 h, the bottles were kept at room temperature for 48 h. Next, after adding 800 mL of deionized water to each bottle, the bottles were stirred with a glass rod until the salt was dissolved and left at room temperature for 24 h. The supernatants in the bottles were then filtered using a vacuum system with cellulose nitrate membrane filters (diameter: 47 mm, pore size: 5 μm), kept in sterile Petri dishes, and dried out at room temperature. The membrane filter was placed in a 100 mL glass bottle and treated with 10–15 mL of 4.4 M NaI. The bottles were sonicated using Ultrasonic Cleaner (WUC-D22H, Witeg, Germany) at 50 Hz for 5 min and centrifuged (Z 32 HK, HERMLE Labortechnik, Germany) at 500×g for 1 min to ensure the separation of microplastics. The supernatants were filtered through cellulose nitrate membrane filters (diameter: 47 mm, pore size: 5 μm) using a vacuum system for microscopic examination (Olympus CX41, Japan) at 4× to 100× magnification. The residual material at the bottom of the bottles was also transferred to clean Petri dishes for microscopic investigation.; Data Quality Method: Several control measures were strictly employed during the study. Glass or metal items were used rather than plastic to avoid contamination. All sampling equipment was cleaned and kept in a storage box before use. All buckets and jars were covered with lids to prevent microplastic fibers from blowing into the containers. The sampling team used masks, gloves, and garments made of natural fibers rather than synthetic clothing. All liquids (deionized water, hydrogen peroxide, etc.) were filtered (5 μm) before use to avoid contamination in the laboratory. All the glassware, containers, filtration units, and other necessary instruments were also cleaned with filtered water. When samples were unused, they were instantly covered with aluminum foil paper. Furthermore, to cross-check the airborne contamination, 500 mL of filtered distilled water in a 1 L glass beaker was kept at three places in the lab for one week during the sample processing and extraction method. The blank control samples were then filtered and observed under a microscope. Any particles identified were examined using Fourier Transform Infrared Spectroscopy (FTIR). Finally, the value was subtracted from each sample to remove the error due to airborne contamination. A representative number of mesoplastics and microplastics (> 300 μm to 25 mm items) from each morphotype were randomly selected to identify the plastic polymer types and analyzed with a FTIR (FTIR-4600, JASCO Inc., Japan) equipped with an Attenuated Total Reflection (ATR) unit. The spectrum range used to identify each plastic was 4000 to 400 cm−1 with a spectral resolution of 4 cm−1. Sixteen co-scans were done for each measurement, and a background measurement was done before measuring each particle. The JASCO Spectra Manager™ II software was used to collect data. Using the KnowItAllR Informatics System 2018, the obtained spectrum was compared to the polymer spectrum from commercial libraries (JASCO Edition, USA). Each particle was analyzed several times, with the results chosen based on the spectrum’s accuracy compared to the reference library..
- Parameter or Variable: microplastic concentration (measured); Units: pieces/m3; Observation Category: in situ; Sampling Instrument: Trowel.
- Parameter or Variable: microplastic concentration (measured); Units: pieces/kg; Observation Category: in situ; Sampling Instrument: Trowel.
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